1. Field of the Invention
The invention relates to a coextruded, biaxially oriented polyester film having at least one matt side and composed of at least one base layer (B) which comprises at least 70% by weight of thermoplastic polyester, and of, applied to the base layer (B), at least one matt outer layer (A). The invention further relates to the use of this film and to a process for its production.
2. Description of the Related Art
The outer layer (A) of the invention has a characteristic matt surface and characteristic optical properties and has good suitability for use as a packaging film or for industrial applications.
The packaging industry has a high requirement for transparent, high-gloss plastic films, e.g. biaxially oriented polypropylene films or biaxially oriented polyester films. In addition, there is increasing requirement for transparent films of this type in which at least one surface layer is not high-gloss but has a characteristic matt appearance giving, for example, the packaging an appearance which is particularly attractive and therefore effective for promotional purposes.
EP 347 646 describes a biaxially oriented polyester film which has at least one outer layer (A) which comprises a filler at a concentration of from 0.5 to 50%, the diameter of this filler having a particular relationship to the thickness of the outer layer. The outer layer also has a certain thickness and has a certain degree of crystallinity determined with the aid of Raman spectroscopy. The topography of the outer layer A makes the film particularly suitable for magnetic recording tapes. The specification gives no information about the gloss achieved for the outer layer A. A film produced (Example 1) in accordance with EP 347 646 did not have the desired matt surface. The gloss of this surface is outside the range claimed in this specification.
EP-A-0 053 498 describes a biaxially oriented polyester film having more than one layer and having a transparent base layer and, on at least one side of this layer, another layer of matt appearance. The layer of matt appearance essentially consists of an ethylene terephthalate copolyester whose copolymer comprises from 1 to 20 mol % of
H(xe2x80x94OCH2CH2xe2x80x94)nOH or
H(xe2x80x94OCH2CH2xe2x80x94)n-1Oxe2x80x94C6H4xe2x80x94Oxe2x80x94(CH2xe2x80x94CH2Oxe2x80x94)n-1H or
H(xe2x80x94OCH2CH2xe2x80x94)n-1Oxe2x80x94C6H4xe2x80x94Xxe2x80x94C6H4xe2x80x94Oxe2x80x94(CH2xe2x80x94CH2Oxe2x80x94)n-lH
(n being an integer from 2 to 140, and X being xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94 or xe2x80x94SO2xe2x80x94) and inert organic particles with median diameter from 0.3 to 20 xcexcm at a concentration of from 3 to 40%, based on the layer of matt appearance. The film has a high level of mattness (gloss below 15), and transparency which is within the bounds of acceptability for specific applications (above 60%). A disadvantage of this film is that if the structure is ABA it is not printable and if the structure is AB it is not processible (on high-speed machinery). In addition, the film has shorcomings in production and its haze level is too high for many applications.
Likewise known from the prior art are matt, biaxially oriented polyester films of milky appearance.
DE-A 23 53 347 describes a process for producing a milky polyester film having one or more layers, which comprises preparing a mixture made from particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film, and orienting the same by orientation in directions running perpendicular to one another, and heat-setting the film. A disadvantage of the process is that it is not possible to reuse the cut material arising during production of the film (essentially a mixture of polyester and ethylene copolymer or propylene copolymer) as regrind without yellowing of the film. This makes the process uneconomic, and the yellowish film produced with regrind was unacceptable to the market. On increasing the concentration of the copolymer in the polyester, the film generally loses its milky character and becomes white with high opacity.
U.S. Pat. No. 3,154,461 claims a process for producing a biaxially oriented matt-surface film from thermoplastic (e.g. polyethylene terephthalate, polypropylene), in which the film comprises incompressible particles (e.g. calcium carbonate, silicon dioxide) with size of from 0.3 to 20 xcexcm and at a concentration of from 1 to 25%. The application concerned also claims the matt film produced by the process given therein. However, the haze level of the film produced by the process of the U.S. patent is too high for many applications.
It was therefore an object of the present invention to provide a biaxially oriented polyester film having at least one matt side and not having the disadvantages of the prior art films mentioned, and having in particular an excellent degree of mattness together with good transparency, very high ease of production and very good processibility. In addition, care should be taken that the film can also be processed on high-speed machinery. It should moreover be ensured that cut material occurring during film production can be reintroduced into the production process as regrind during production of the film in amounts of up to 60% by weight, based on the total weight of the film, without any significant adverse effect on the physical or optical properties of the film.
According to the invention, the object is achieved by providing a biaxially oriented polyester film having at least one matt side and having at least one base layer (B) which comprises at least 70% by weight of thermoplastic polyester and has at least one matt outer layer (A), where the matt outer layer (A) comprises a pigment system which is characterized by the following features
a) the median diameter of the pigment grains (d50) is in the range from 2 to 5 xcexcm
b) the spread of the distribution of the grain size of the pigment grains, expressed via the SPAN 98, is less than or equal to 1.9, and
c) the concentration of the pigment grains is in the range from 1.0 to 10% by weight.